Stratospheric and tropospheric components of 7Be in surface air

Abstract

7Be and 90Sr concentrations were measured simultaneously in samples from NASA's Global Atmospheric Sampling Program. The samples were collected from January 1978 through June 1979 at 9 to 12 km and between 11° and 60°N. 7Be concentrations ranged from 150 to 6900 fCi/m3, while 90Sr correspondingly varied from 10 to 320 fCi/m3. The average 7Be concentration in the samples collected in the lower stratosphere was 4500 fCi/m3. In the upper troposphere, 7Be concentrations were generally less than 1000 and often only a few hundred fCi/m3. In situ production of 7Be in the upper troposphere at ∼11 km was estimated at 340 fCi/m3. Since 90Sr is only present in the atmosphere because of nuclear testing, except immediately after such tests, 90Sr in the troposphere is due only to the subsidence of stratospheric air. Since atmospheric processes are not expected to alter 7Be/90Sr ratios during transport, surface 90Sr and stratospheric 7Be/90Sr ratios can be used to estimate 7Be transported from the stratosphere. Our analyses show that on an annual basis the stratosphere contributed ∼25% of the observed 7Be concentration. During spring and summer months, the stratospheric component can approach 40%, while on a daily basis this can be even larger.